Where reinforced concrete is in contact with seawater or diluted seawater (brackish water), such as in pilings or support structures for bridges, dock structures, or seawater canals, it is subject to a very high rate of corrosion. Corrosion is accelerated in this area due to high salt concentration and because of the availability of oxygen for the cathodic reaction. The availability of oxygen is especially important since the rate of corrosion is often cathode limited. Conditions of alternate wet and dry cycles therefore create an ideal situation for rapid corrosion.
This has recently been recognized as a significant problem, but no successful system for the cathodic protection of reinforcing steel has yet been developed for structures in contact with seawater. The major obstacles to the use of cathodic protection in this case has been the leakage of the impressed current into the seawater.
The most critical area is the area of very high corrosion near the tide level. The area above high tide level also undergoes corrosion, and it is desirable to apply cathodic protection to this portion of the structure as well.
In the past, anodes such as conductive paints have been applied to the outside of such columns. These efforts were unsuccessful since the apPlied current is easily leaked or diverted into the seawater during periods of high tide or when the structure is subjected to waves or swells. This occurs since full strength seawater has a specific resistivity of about 20 ohm-cm, providing a much more conductive path to ground than the concrete in the structure, which has a specific resistivity of 5000-15,000 ohm-cm. If a substantial amount of current is leaked to the seawater rather than being directed into the concrete structure, the reinforcing steel will not be cathodically protected. If on the other hand, the anodes are placed high enough on the structure that current leakage does not occur, the critical area of highest corrosion will not be protected.
In order to leak significant amounts of current to the seawater, direct contact between the anode and seawater need not occur. Current can travel a short distance through the concrete near the structure surface and then into the seawater, again avoiding the reinforcing steel and providing no cathodic protection.
This process results not only in ineffective protection, but also in damage to the anodes and to the anode-concrete interface. This damage occurs because the anodes and interface are specifically designed not to exceed a current density of 10 mA/ft.sup.2 of anode surface. A higher current density is known to shorten anode lifetime and to generate sufficient acid to damage the concrete near the anode surface. When current leakage occurs, an area of very high current will be present near the seawater level, causing anode and acid damage.